This document discusses the structure and function of the kidney. It describes the basic unit of the kidney, the nephron, and its role in filtration, reabsorption, and secretion. Specifically, it explains how glomerular filtration occurs to remove waste from the bloodstream and how certain transport proteins and hormones like antidiuretic hormone and aldosterone regulate water and electrolyte balance. Kidneys are essential excretory organs that exist in most animal species to maintain homeostasis.